In the field of information recording, there has been ongoing research on optical information recording. The optical information recording has a number of advantages, including the ability to record and reproduce information non-contact, and compatibility to different memory formats, including read-only, write-once, and rewritable memories, in addition to inexpensively realizing large storage media. Thus, the optical information recording is expected to have a wide variety of applications, including industrial and consumer applications.
A recent research trend for optical disk devices using optical information recording has been to increase the information recording capacity per unit area of the de facto standard optical disks (for example, optical disks with a 120 mm diameter, including CD and DVD), and to realize smaller optical disks and optical disk devices.
The information recording capacity per unit area can be increased by reducing the wavelength of a light source, or by increasing the numerical aperture (NA) of an objective lens.
The recent development of a blue laser has made a great contribution in reducing the wavelength of a light source. However, a further reduction has been impeded by the absorption of light by the optical components. Common materials of the optical components are glass and plastic. As a general rule, materials with good workability (low melting point) have a longer wavelength of absorption limit. For example, plastics, which have a lower melting point than common glass and provide better workability, have a wavelength of absorption limit in a range of 300 nm to 400 nm, which is longer than that of common glass ranging from 200 nm to 300 nm. Thus, in mass-produced optical components using plastic (or glass with good workability), the light source is limited to a particular range of wavelength that can be absorbed.
As to increasing the NA of a lens, the NA can be increased by designing. However, in a lens with a high NA, the distance between the objective lens and the optical disk (“working distance” hereinafter) is as short as about 0.1 mm, and at most about 0.5 mm, which is shorter than that in conventional lenses of CDs and DVDs.
Thus, the objective lens and the optical disk collide when vibrations or other disturbances caused when the device is used in mobile applications move the objective lens out of the proper position determined by the focusing servo control. As a result, the surface of the objective lens is damaged, impairing optical characteristics, such as transmittance of the objective lens or aberration characteristics.
Japanese Publication for Unexamined Patent Application No. 67700/2001 (Tokukai 2001-67700) (published on Mar. 16, 2001) (“Publication 1” hereinafter) discloses a technique for solving such a problem. As illustrated in FIG. 8, this publication proposes a structure including an objective lens 66 composed of a first lens 61 and a second lens 62, and a lens holder 63 holding the first lens 61 opposite an optical disk (not shown), wherein the lens holder 63 has a shock absorber 63a that projects out of the surface of the first lens 61 toward the optical disk. Note that, indicated by the reference numeral 64 is the holder for a second lens 62.
With this structure, even when external vibrations or other disturbances move the objective lens 66 out of the proper position determined by the focusing servo control (not shown), only the shock absorber 63a is in contact with the optical disk and the first lens 61 does not directly collide with the optical disk. As a result, damage to the first lens 61 is prevented, thereby preventing deterioration of optical characteristics, such as transmittance of the objective lens 60 or aberration characteristics, caused when the objective lens 60 is damaged.
Japanese Publication for Unexamined Patent Application No. 258336/1993 (Tokukaihei 5-258336) (published on Oct. 8, 1993) (“Publication 2” hereinafter) discloses a technique for protecting the surface of the objective lens from dirt such as dust.
FIG. 9 illustrates a structure disclosed in this publication. As illustrated in FIG. 9, the publication provides an antistatic, oil-repellent, and lubricative protective-coating 71 on the surface of an objective lens 69 facing an optical disk 77. Such an antistatic coating is generally provided by ITO, for example.
With this structure, dust does not contaminate the objective lens 69, thereby preventing deterioration of optical characteristics, such as transmittance of the objective lens 69 or aberration characteristics.
Another problem of the collision of the objective lens and the optical disk is that the surface of the objective lens is electrified by the static charge generated by the collision. This is even more problematic than physically damaging the objective lens and/or the optical disk.
The charge on the surface of the objective lens attracts dust, and the dust or other particles adhered on the surface of the objective lens impairs the optical characteristics of the objective lens. Further, when the objective lens collides again with the optical disk, the dust or particles damage the objective lens and the optical disk. This causes a secondary problem that error is caused in recording or reading signals (information) in and from the optical disk.
The problem is not solved by the technique disclosed in Publication 1. In fact, Publication 1 does not even consider that the shock absorber 63a (see FIG. 8) becomes electrified by the generated charge of the collision with the optical disk, or that the generated charge attracts dust, nor does it consider the adverse effect of dust on the optical pickup. This is clear from the fact that Publication 1 does not even indicate that the shock absorber 63a serves to prevent charge, even though it does disclose examples of a material for the shock absorber 63a, including elastic materials such as rubber and felt, and resin.
That is, the collision of the shock absorber and the optical disk does not damage the objective lens 66 (first lens 61 in particular) but the shock absorber 63a is statically charged when it collides with the optical disk rotating at a high speed, thereby charging the surface of the shock absorber 63a. The charged surface of the shock absorber 63a attracts dust, and the dust or particles adhered to the surface of the shock absorber 63a are likely to damage the optical disk when the shock absorber 63a and the optical disk collide again. The problem becomes particularly serious when an objective lens 66 with a high NA is used, because in this case the damaged surface of the optical disk causes noise and generates errors in recording or reading information in and from the optical disk.
This is expected to become even more problematic in the future when the information recording density of the optical disk is increased and the working distance is reduced to cause the optical disk and the shock absorber 63a to collide more frequently.
In the optical system disclosed in Publication 2, the provision of the protective coating 71 having an antistatic property on the surface of the objective lens 69 facing the optical disk 77 prevents dust or particles from contaminating the objective lens 69. However, this system assumes that the objective lens 69 and the optical disk 77 are distanced from each other, and does not even consider the possibility that the optical disk 77 collides with the protective coating 71.
Thus, when the optical disk 77 frequently collides with the protective coating 71 as in the case where the objective lens 69 has a high NA, it is highly likely that the protective coating 71 is detached, with the result that the effect of the protective coating 71 as a protective film, protecting the objective lens 69 from dust, is quickly lost. The protective coating 71, when it is completely detached or even partly detached and remains on the surface of the objective lens 69, influences the focusing characteristics of the objective lens 69 with a high NA. That is, a sufficiently small focusing spot cannot be obtained.
Publication 1 and Publication 2 may be combined to obtain a structure in which the shock absorber is provided in the vicinity of the objective lens with the protective coating. However, even in this structure, the shock absorber is still charged when the optical disk and the shock absorber collide with each other, and the generated charge attracts dust or particles to the shock absorber. As a result, the optical disk is damaged when the shock absorber collides with the optical disk again. When the objective lens has a NA of 0.8 or greater, even a small scratch on the optical disk causes errors in recording and reading information.
That is, damage to the optical disk and the shock absorber still occurs in a structure combining the conventional techniques disclosed in Publication 1 and Publication 2.